The present invention concerns an improved process for supplying metal ions to a bath for electroplating a nickel alloy or cobalt alloy for the purpose of replenishing nickel or cobalt ions consumed in the electroplating step. The invention concerns also an apparatus for carrying out the process.
Nickel alloy electroplating or electroplating of a combination of nickel and a base metal, such as zinc-nickel or tin-nickel, has been widely practiced to obtain high corrosion resistance. Also practiced is cobalt alloy electroplating or electroplating of a combination of cobalt and a base metal, such as zinc-cobalt. Since the nickel alloy electroplating and cobalt alloy electroplating are quite similar technologies, the following explanation is given on the nickel alloy electroplating.
As the result of progress of electroplating ions of nickel and the mating metal in the electroplating solution will inevitably be consumed and the cations or metal ions are replaced with hydrogen ions, and the pH of the electroplating solution will decrease. When replenishing metal ions to the electroplating bath, base metals such as zinc and tin may be ionized by chemical reactions when metal pellets are charged into the solution. On the other hand, nickel, a corrosion-resistant metal, may not be ionized by simply charging the metal pellets into the solution having the decreased pH, and therefore, replenishment of nickel ions must be realized by some other methods. The conventional method of replenishing nickel ions to a nickel alloy electroplating solution is carried out by dissolving nickel carbonate.
Nickel carbonate is relatively expensive, and it is difficult to obtain a product of high purity. Commercially available products are called "basic nickel carbonate" and contain, in addition to nickel carbonate, not only nickel hydroxide but also sodium carbonate. Use of low purity nickel carbonate may result in low quality in electroplated products by unbalancing of pH or by invasion of undissolved nickel hydroxide (solubility of which is low) into the electroplating line.
As one of the breakthroughs of this problem, it was proposed to ionize metallic nickel by electrolysis and replenish the nickel ions to the electroplating bath (see for example, Japanese Patent Disclosures No. 4-13900 and No. 6-25900).
In these methods electrolysis is carried out by using an anode of metallic nickel and a cathode of a material having a low hydrogen overpotential such as a noble metal of the platinum group, and hydrogen gas evolves from the cathode. In other words, deposition of nickel on the cathode, which is a principal reaction, is prevented by giving priority to hydrogen gas generation reaction at the cathode so as to have the nickel ions retained in the solution.
Even though a substance which has a low hydrogen over-potential and is active to generation of hydrogen gas is used as the material of cathode, it is practically impossible to completely suppress deposition of a metal such as nickel over a long period of time. Therefore, it is tried to redissolve the deposited nickel by application of electric current in the reverse polarity so as to heighten the recovery percentage of nickel. However, repeated reverse application of electric current causes dissolving out of the cathode surface layer having high catalytic activity for the hydrogen generation reaction, and initial performance of the electrode is lost. Thus, life of the electrode is not as long as acceptable in practical use.